This invention relates to arrangements for reducing electromagnetic interference.
The invention is more particularly concerned with arrangements for reducing interference in electrical equipment such as, for example, avionics units.
In many environments, such as that of an aircraft or ship, there can be a large amount of radio frequency interference from electrical apparatus in the vicinity. Avionics units and the like can also be susceptible to interference from lightning and from electromagnetic pulses caused by nuclear explosions. In order to ensure the reliability of avionic equipment and to ensure that it is available for use during flight, it is important that such interference is reduced to a minimum. This is conventionally achieved by screening the equipment casing and cables connected with the equipment, and by the use of filters.
Screened cables are, however, generally undesirable because of their increased weight and size. Also, they can be prone to corrosion and require complicated connections at their ends. The use of filters which obviate the need to screen cables is therefore advantageous. Such filters, can be incorporated into a connector, such as described in patent application GB No. 2025158A. Such connectors include an individual filter element around each connector pin which supports the pin in a ground plane. These filter elements, however, are very fragile and tend to become broken during use leading to failure of the filter. In a multi-pin connector, failure of one of the filter elements can lead to radio frequency interference passing into the equipment housing and degrading the signals on all the other lines.
In avionics units there may be as many as four hundred individual connector pins each of which has to be filtered. This leads to a very high cost of the filter-connector and increases the risk of interference entering the housing through a failed filter.